Embodiments of the invention are directed, in general, to communication systems and, more specifically, to pilot design used in communications systems.
The global market for both voice and data communication services continues to grow as does users of the systems which deliver those services. As communication systems evolve, system design has become increasingly demanding in relation to equipment and performance requirements. Future generations of communication systems, will be required to provide high quality high transmission rate data services in addition to high quality voice services. Orthogonal Frequency Division Multiplexing (OFDM) is a technique that will allow for high speed voice and data communication services.
Orthogonal Frequency Division Multiplexing (OFDM) is based on the well-known technique of Frequency Division Multiplexing (FDM). OFDM technique relies on the orthogonality properties of the fast Fourier transform (FFT) and the inverse fast Fourier transform (IFFT) to eliminate interference between carriers. At the transmitter, the precise setting of the carrier frequencies is performed by the IFFT. The data is encoded into constellation points by multiple (one for each carrier) constellation encoders. The complex values of the constellation encoder outputs are the inputs to the IFFT. For wireless transmission, the outputs of the IFFT are converted to an analog waveform, up-converted to a radio frequency, amplified, and transmitted. At the receiver, the reverse process is performed. The received signal (input signal) is amplified, down converted to a band suitable for analog to digital conversion, digitized, and processed by a FFT to recover the carriers. The multiple carriers are then demodulated in multiple constellation decoders (one for each carrier), recovering the original data. Since an IFFT is used to combine the carriers at the transmitter and a corresponding FFT is used to separate the carriers at the receiver, the process has potentially zero inter-carrier interference.
FIG. 1 is a diagram illustrative of the Frequency 103—Time 101 Representation 100 of an OFDM Signal. In FDM different streams of information are mapped onto separate parallel frequency channels 140. Each FDM channel is separated from the others by a frequency guard band to reduce interference between adjacent channels.
The OFDM technique differs from traditional FDM in the following interrelated ways:    1. multiple carriers (called sub-carriers 150) carry the information stream;    2. the sub-carriers 150 are orthogonal to each other; and    3. a Cyclic Prefix (CP) 110 (also known as guard interval) is added to each symbol 120 to combat the channel delay spread and avoid OFDM inter-symbol interference (ISI).
The data/information carried by each sub-carrier 150 may be user data of many forms, including text, voice, video, and the like. In addition, the data includes control data, a particular type of which is discussed below. As a result of the orthogonality, ideally each receiving element tuned to a given sub-carrier does not perceive any of the signals communicated at any other of the sub-carriers. Given this aspect, various benefits arise. For example, OFDM is able to use orthogonal sub-carriers and, as a result, thorough use is made of the overall OFDM spectrum. As another example, in many wireless systems, the same transmitted signal arrives at the receiver at different times having traveled different lengths due to reflections in the channel between the transmitter and receiver. Each different arrival of the same originally-transmitted signal is typically referred to as a multi-path. Typically, multi-paths interfere with one another, which is sometimes referred to as InterSymbol Interference (ISI) because each path includes transmitted data referred to as symbols. Nonetheless, the orthogonality implemented by OFDM considerably reduces ISI and, as a result, often a less complex receiver structure, such as one without an equalizer, may be implemented in an OFDM system.
A Cyclic Prefix (CP) (also known as guard interval) is added to each symbol to combat the channel delay spread and avoid OFDM inter-symbol interference (ISI). FIG. 2 is a diagram illustrative of using Cyclic Prefix (CP) to eliminate ISI and perform frequency domain equalization. Blocks 200 each comprising cyclic prefix 210 coupled to data symbols 220 to perform frequency domain equalization. OFDM typically allows the application of simple, 1-tap, frequency domain equalization (FDE) through the use of a Cyclic Prefix (CP) 210 at every FFT processing block 200 to suppress multi-path interference. Two blocks are shown for drawing convenience. CP 210 eliminates inter-data-block interference and multi-access interference using Frequency Division Multiple Access (FDMA).
Since orthogonality is guaranteed between overlapping sub-carriers and between consecutive OFDM symbols in the presence of time/frequency dispersive channels, the data symbol density in the time-frequency plane can be maximized and high data rates can be very efficiently achieved for high Signal-to-Interference and Noise Ratios (SINR).
FIG. 3 is a diagram illustrative of Cyclic Prefix (CP) Insertion. A number of samples is typically inserted between useful OFDM symbols 320 (guard interval) to combat OFDM ISI induced by channel dispersion, assist receiver synchronization, and aid spectral shaping. The guard interval 310 is typically a prefix that is inserted 350 at the beginning of the useful OFDM symbol (OFDM symbol without the CP) 320. The CP duration 315 should be sufficient to cover most of the delay-spread energy of a radio channel impulse response. It should also be as small as possible since it represents overhead and reduces OFDM efficiency. Prefix 310 is generated using a last block of samples 340 from the useful OFDM symbol 330 and is therefore a cyclic extension to the OFDM symbol (cyclic prefix).
When the channel delay spread exceeds the CP duration 315, the energy contained in the ISI should be much smaller than the useful OFDM symbol energy and therefore, the OFDM symbol duration 325 should be much larger than the channel delay spread. However, the OFDM symbol duration 325 should be smaller than the minimum channel coherence time in order to maintain the OFDM ability to combat fast temporal fading. Otherwise, the channel may not always be constant over the OFDM symbol and this may result in inter-sub-carrier orthogonality loss in fast fading channels. Since the channel coherence time is inversely proportional to the maximum Doppler shift (time-frequency duality), this implies that the symbol duration should be much smaller than the inverse of the maximum Doppler shift.
The large number of OFDM sub-carriers makes the bandwidth of individual sub-carriers small relative to the total signal bandwidth. With an adequate number of sub-carriers, the inter-carrier spacing is much narrower than the channel coherence bandwidth. Since the channel coherence bandwidth is inversely proportional to the channel delay spread, the sub-carrier separation is generally designed to be much smaller that the inverse of the channel coherence time. Then, the fading on each sub-carrier appears flat in frequency and this enables 1-tap frequency equalization, use of high order modulation, and effective utilization of multiple transmitter and receiver antenna techniques such as Multiple Input/Multiple Output (MIMO). Therefore, OFDM effectively converts a frequency-selective channel into a parallel collection of frequency flat sub-channels and enables a very simple receiver. Moreover, in order to combat Doppler effects, the inter-carrier spacing should be much larger than the maximum Doppler shift.
The baseband representation 400 of the OFDM signal generation using an N-point IFFT 460 is shown in FIG. 4, where n refers to the nth sub-channel modulated data symbol 420, during the time 401 period 0<t≦Tu where Tu is OFDM useful symbol duration. The vector S is defined as the useful OFDM symbol and is practically the time superposition of the N narrowband modulated sub-carriers. Therefore, from a parallel stream of N data sources, a waveform composed of N orthogonal sub-carriers 407 is obtained. At the receiver, a computationally efficient Fast Fourier Transform (FFT) may be used to demodulate the multi-carrier information and to recover the transmitted data.
FIG. 5 shows the concepts of frequency diversity 500 and multi-user diversity 505. Using link adaptation techniques based on the estimated dynamic channel properties, the OFDM transmitter can adapt the transmitted signal to each User Equipment (UE) to match channel conditions and approach the ideal capacity of frequency-selective channel. Thanks to such properties as flattened channel per sub-carrier, high-order modulation, orthogonal sub-carriers, and MIMO; it is possible to improve spectrum utilization and increase achievable peak data rate in OFDM system. Also, OFDM can provide scalability for various channel bandwidths (i.e. 1.25, 2.5, 5, 10, 20 MHz) without significantly increasing complexity.
OFDM may be combined with Frequency Division Multiple Access (FDMA) in an Orthogonal Frequency Division Multiple Access (OFDMA) system to allow multiplexing of multiple UEs over the available bandwidth. Because OFDMA assigns UEs to isolated frequency sub-carriers, intra-cell interference may be avoided and high data rate may be achieved. The base station (or Node B) scheduler assigns physical channels based on Channel Quality Indication (CQI) feedback information from the UEs, thus effectively controlling the multiple-access mechanism in the cell. For example, in FIG. 5, transmission to each of the three UEs 501, 502, 503 is scheduled at frequency sub-bands where the channel frequency response allows for higher SINR relative to other sub-bands. This is represented by the Received signal levels R501, R502, and R503 for users 501, 502 and 503 at Frequencies F501, F502, and F503 respectively.
OFDM can use frequency-dependent scheduling with optimal per sub-band Modulation & Coding Scheme (MCS) selection. For each UE and each Transmission Time Interval (TTI), the Node B scheduler selects for transmission with the appropriate MCS a group of the active UEs in the cell, according to some criteria that typically incorporate the achievable SINR based on the CQI feedback. In addition, sub-carriers or group of sub-carriers may be reserved to transmit pilot, signaling or other channels. Multiplexing may also be performed in the time dimension, as long as it occurs at the OFDM symbol rate or at a multiple of the symbol rate (i.e. from one IFFT computation to the next). The MCS used for each sub-carrier or group of sub-carriers can also be changed at the corresponding rate, keeping the computational simplicity of the FFT-based implementation. This allows 2-dimensional time-frequency multiplexing, as shown in FIG. 6 and FIG. 7.
Transmission Time Interval (TTI) may also be referred to as a frame.
Turning now to FIG. 6, which is a diagram illustrative of a configuration for multi-user diversity. The minimum frequency sub-band used for frequency-dependent scheduling of a UE typically comprises of several sub-carriers and may be referred to as a Resource Block (RB) 620. Reference number 620 is only pointing to one of the 8 RBs per OFDM symbol shown as example and for drawing clarity. RB 620 is shown with RB bandwidth 625 in frequency dimension and TTI duration 610 in time dimension. Each RB may be comprised of continuous sub-carriers and thus be localized in nature to afford frequency-dependent scheduling. A high data rate UE may use several RBs within same TTI 630. UE #1 is shown as an example of a high rate UE. Low data rate UEs may be multiplexed within the same RB 640.
Alternatively referring to FIG. 7, which is a diagram illustrative of a configuration for frequency diversity, an RB 720 may correspond to a number of sub-carriers substantially occupying the entire bandwidth thereby offering frequency diversity. This may be useful in situations where CQI feedback is not available or it is unreliable (as is the case for high speed UEs).
To facilitate data-aided methods, OFDM systems periodically insert reference (or pilot) symbols that are known a priori, into the transmission signal. The receiver can thus estimate the channel response based on the received pilot symbols and the known transmitted pilot symbols. In an OFDM based communication system, pilot symbols are transmitted in addition to data symbols in order to serve, inter aila, in providing a reference for the receiver to estimate the channel medium and accordingly demodulate the received signal. A pilot signal also referred to as reference signal is composed of the pilot symbols.
The DownLink (DL) pilot signal should provide effective performance for the following functions:                Channel estimation at all possible operating carrier frequencies for all physical channels for all channel multi-path delay spreads (frequency selectivity) encountered in practice and for all UE speeds of interest.        CQI measurement for link adaptation and channel-dependent scheduling.        Sector identification of sector within the same cell.        Measurements for cell search and handover.        
UE dedicated pilot signals may also be used for UE-dependent adaptive beam-forming. Moreover, as the pilot signal is actually overhead consuming resources that could have been otherwise dedicated for data transmission, it should have minimum time/frequency and power overhead.
Two types of pilot structure have been previously examined;                i) a Time-Division Multiplexed (TDM) pilot structure where the pilot signal is placed on a single OFDM symbol per TTI (FIG. 8).        ii) a scattered pilot structure where the pilot signal is placed in every OFDM symbol per TTI (FIG. 9)        
In the example shown in FIG. 8, the TTI is assumed to comprise of seven OFDM symbols 810. Moreover, the TTI duration is assumed to be 0.5 milliseconds. Shaded dots indicate pilot locations for 1 transmit antenna. In the example of FIG. 9, the TTI is assumed to comprise of seven OFDM symbols 910. Shaded dots indicate pilot locations for 1 transmit antenna. Channel estimation is based on time and frequency interpolation among pilot sub-carriers in order to obtain the channel estimates at the position corresponding to data sub-carriers. In order to be able to perform frequency interpolation, the pilot sub-carrier spacing in the frequency domain should be smaller than the 50% correlation coherence bandwidth of the channel for all channels of interest. Similarly, in order to be able to perform time interpolation, the pilot sub-carrier spacing in the time domain should be smaller than the 50% coherence time of the channel at the operating carrier frequency for all UE speeds of interest. The example shown in FIG. 8 has a spacing in frequency domain ΔFpil 830 of 4 sub-carriers and spacing in time domain ΔTpil 820 of 7 OFDM symbols. The example shown in FIG. 9 has a spacing in frequency domain ΔFpil 930 of 4 sub-carriers and spacing in time domain ΔTpil 920 of 1 OFDM symbol.
Additional requirements for the pilot signal design may relate to the ability to demodulate only an initial sub-set of the TTI without having to receive the entire TTI. This is applicable, for example, when the control channel associated with scheduling of UEs in the current TTI at various RBs is transmitted in the first few OFDM symbols in every TTI. Then, it may be beneficial to demodulate and decode the control channel prior to the reception of the remaining OFDM symbols in the referenced TTI in order to reduce latency. Moreover, in order to improve channel estimation performance, it is desirable to capture as much of the transmitted pilot signal power as possible without additional latency. Clearly, the pilot signal power from preceding TTIs may be assumed available to the UE but the UE will have to incur additional decoding latency if it were to obtain the pilot signal power from succeeding TTIs. However, this would be particularly desirable for channel estimation performance as it would result to pilot signal availability that is more symmetric relative to the TTI of interest.
Based on the above discussion, the following disadvantages can be directly identified for the pilot structures of prior art:                1) The TDM pilot structure cannot provide reliable channel estimation and communication support at high UE speeds. For example, for a UE speed of 250 Kmph and carrier frequency of 2.6 GHz, the channel at the fourth OFDM symbol in the TTI structure of FIG. 8 has very little correlation with the channel at the first OFDM symbol of the same TTI or the channel at the first OFDM symbol of the next TTI where the pilot sub-carriers are located. The same problem would persist if the pilot sub-carriers were placed at another OFDM symbol in the TTI (not the first symbol).        2) The nature of the scattered pilot structure results to very little pilot signal power concentration per OFDM symbol. As a result, if the control channel is transmitted in the first few OFDM symbols in the TTI, there may not be enough pilot signal power to demodulate is prior to the reception of the entire TTI. Moreover, if any substantial pilot signal power from the succeeding TTI is to be obtained in order to provide some symmetry in the channel estimated for the TTI of interest, substantial latency will be incurred as the pilot sub-carriers in several OFDM symbols of the succeeding TTI will need to be captured.        
There is a need for an improved pilot structure design in order to achieve accurate channel estimates for high user equipment (UE) speeds in mobile operations while also achieve the ability to use substantial pilot energy from succeeding TTI with minimum latency.